1. Field of the Invention
The invention is directed to compositions and processes useful for the roll-to-roll manufacturing of electrophoretic and electromagnetophoretic displays.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO02/65215), all of which are incorporated herein by reference. The improved EPD comprises closed cells formed from microcups of well-defined shape, size and aspect ratio, filled with charged pigment particles dispersed in a dielectric solvent and sealed with a polymeric sealing layer.
An electromagnetophoretic display (EMPD) technology was recently disclosed in No. 60/367,325 filed on Mar. 21, 2002, the content of which is incorporated herein by reference. It is understood that while not specifically mentioned, the scope of the present invention encompasses both EPDs and EMPDs.
Traditionally, the EPDs are manufactured by laminating both sides of a compartment filled with an electrophoretic fluid with, patterned electrode layers (such as ITO films). The dispersion-filled compartment may be one of those discussed above, such as that of the partition type or one that involves microcapsules or microchannels. The two electrode layers, however, must be offset in order to expose the conductor lines or patterns for connection to the driver circuitry. The process involving such asymmetric lamination typically is a batch-wise process. In other words, the display panels are laminated piece by piece; thus the process has a very low throughput.
In the manufacturing of a display involving microcups, generally a layer of embossable thermoplastic or thermoset resin composition is first coated on a substrate layer, followed by embossing of the thermoplastic or thermoset resin composition to form the microcups. The microcups are then filled with an electrophoretic or electromagnetophoretic fluid and sealed with a sealing layer.
A second substrate layer is then laminated over the filled and sealed microcups. The display has at least one electrode layer and the electrode layer is usually a conductive layer coated on the side of the substrate layer facing the filled microcups. In addition, at least one of the two substrate layers is transparent to the viewer.
For an EPD, there are two electrode coated substrate layers and the display cells are sandwiched between the two layers. For an EMPD, there are one substrate layer and one electrode coated substrate layer and the cells are sandwiched between the two layers.
The manufacturing of the display described above may be carried out roll-to-roll and as a result, the display cells sandwiched between the two flexible substrate layers may be produced continuously on a web. To complete the construction of a display device, the electrode lines of the display cells must be exposed and connected to a driver circuitry.
To expose the electrode lines for circuitry connection, a strip coating or patch coating process may be employed to deposit the embossable resin composition onto selected areas of the substrate. However, either process has at least four major disadvantages: (1) a sophisticated coating on the first substrate with precision tracking and metering of the embossing composition is needed to define the edge and also to avoid undesirable resin coverage onto the areas having exposed electrode lines or patterns; (2) uneven edge thickness and profile are produced because the embossable composition tends to be squeezed out side-ways during embossing under pressure; (3) the shape and dimension of the display are predefined by the strip or patch coating process; and (4) the filled and sealed microcups must be cut or sliced to the predetermined dimension and laminated piece by piece with registration to the second substrate.
The precision tracking of the coating fluid is difficult particularly if the space between the embossed and non-embossed areas is narrowed to increase the yield or to reduce the amount of scrap material. The uneven edge thickness and profile tend to cause defects and non-uniformity during the subsequent filling and sealing steps. The lack of format flexibility and incompatibility with the roll-to-roll lamination process are even more undesirable from the view points of labor cost, production yield and throughput.
Accordingly, there is a strong need for a more cost effective process with higher yield and throughput to expose the electrode lines or patterns for connection to the driver circuitry.